1. Field of Invention
The invention relates to a secondary air supply system and, more particularly, to a secondary air supply system including a secondary air supply pipe that is divided into two branch portions each having an air control valve and being connected to each of two exhaust systems of an internal combustion engine with two banks. The invention relates to a secondary air supply system that supplies secondary air to a section upstream of an exhaust emission control device provided in an exhaust passage of the internal combustion engine for detecting an abnormality in components of the secondary air supply system.
2. Description of Related Art
Generally a catalyst is employed for purifying exhaust gas discharged from the internal combustion engine. The catalytic temperature has to be raised from the low-temperature state as quickly as possible upon start-up so as to improve the purifying rate. The catalyst is activated upon increase in the oxygen concentration of the exhaust gas to raise the catalytic temperature. In the aforementioned case, a secondary air supply system is employed to supply air into the exhaust gas so as to be mixed for raising the catalytic temperature. The aforementioned secondary air supply system supplies air from an air pump into the exhaust pipe via a secondary air supply pipe connected thereto.
The catalyst is not activated at an appropriate timing unless secondary air can be supplied when needed. If, however, the secondary air is supplied in a state where the catalyst has been already activated at sufficiently high temperature, the catalyst may be excessively heated. Then the secondary air supply pipe is provided with an air control valve that controls supply of the secondary air such that appropriate quantity of secondary air is supplied to the exhaust pipe.
If the air control valve has an abnormality, appropriate quantity of secondary air cannot be supplied, causing either deterioration in exhaust emission or excessive heating of the catalyst. For example, JP-A-2003-83048 discloses the secondary air supply system that allows diagnosis of abnormality in the air control valve. In the aforementioned system, a pressure sensor is provided between the air control valve and the air pump so as to detect the abnormality in the air control valve based on fluctuation of the pressure detected by the pressure sensor upon operation of the air control valve.
There is an internal combustion engine having a plurality of banks of, for example, two cylinder groups each including a plurality of cylinders like a V-type engine, and each of the banks is provided with an exhaust pipe. In the case where secondary air is supplied to the respective exhaust pipes of the V-type engine, it is possible to supply secondary air to the respective exhaust pipes independently. In this case, however, two air pumps are required, resulting in the cost increase.
In the apparatus disclosed in JP-A-5-86848, secondary air is supplied to two exhaust valves via two branch portions of the secondary air supply pipe from its joint portion. The air control valve is provided for each of the branch portions of the secondary air supply pipe. In this case, the two branch portions of the secondary air supply pipe join at upstream side, and each secondary air flowing through the respective branch portions may influence with each other.
The secondary air supply system that allows abnormality diagnosis of two air control valves accurately at low costs has never been developed (see JP-A-2003-83048).
In the known exhaust emission control device of the internal combustion engine, the catalyst that exhibits oxidizing function is provided within the exhaust passage so as to purify the exhaust gas by reducing the content of the exhaust gas, that is, carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOX). There is a known secondary air supply apparatus in which air is supplied under pressure from the air pump to a secondary air supply passage having a valve connected to the exhaust passage for supplying secondary air into the exhaust pipe so as to increase oxygen concentration. Then HC, CO contained within the exhaust gas are oxidized to improve purification of the exhaust gas. In the aforementioned secondary air supply apparatus, in the case where abnormality is detected in components such as the air pump or the air control valves, the exhaust gas purification rate may be decreased to deteriorate emission. Therefore, the pressure sensor is provided in the secondary air supply passage so as to detect the abnormality in the components based on the pressure value detected by the pressure sensor (see JP-3444458).
If the internal combustion engine includes two banks as aforementioned, the exhaust emission control device is provided for each bank so as to purify the exhaust gas by oxidizing HC and CO contained within the exhaust gas in the exhaust passage from the respective banks. Then the secondary air supply pipe of the secondary air supply system is divided into two branch portions. In this case, an auxiliary air control valve is provided for a joint portion of the secondary air supply pipe upstream of the branch portions, and two air control valves are provided for the respective branch portions.
FIG. 19 shows a timing chart representing an operation of the aforementioned secondary air supply system and a graph representing a relationship between time and pressure with respect to the operation of the secondary air supply system. When the air pump provided on the joint portion of the secondary air supply pipe is driven and the auxiliary air control valve at the upstream side is opened after a predetermined elapse of time, the pressure P within the secondary air supply pipe increases up to P0. When the air control valve provided in one of the branch portions is opened after a predetermined elapse of time, the pressure P is reduced to the pressure P1. When the other air control valve provided in the other branch portion is opened after a predetermined elapse of time, the pressure P is reduced to P2. Then secondary air is supplied to the section of the exhaust passage upstream of the exhaust emission control device. The pressure Pon upon supply of the secondary air is measured for a time period TPon. After measuring the pressure Pon, the pump is stopped, and the firstly and the secondly opened air control valves at the downstream side are closed simultaneously. Then supply of secondary air is stopped. Pressure Poff in the state where secondary air is not supplied after a predetermined elapse of time is measured for a time period TPoff. The auxiliary air control valve, then is closed.
Then it is determined whether there is an abnormality in the components of the secondary air supply system based on the pressure difference ΔP1 between the pressure detected upon opening of the auxiliary air control valve and the pressure detected upon opening of the first air control valve (firstly opened air control valve), that is, (P0−P1), the pressure difference ΔP2 between the pressure detected upon opening of the first air control valve and the (secondly opened air control valve) pressure detected upon opening of the second air control valve, that is, (P1−P2), and the pressure Poff measured upon stop of the secondary air supply system.
As shown in FIG. 19, as the pressure difference ΔP1 upon opening of the first air control valve at the downstream side after opening the auxiliary air control valve at the upstream side is relatively large, the abnormality in the component may be easily detected. Meanwhile as the pressure difference ΔP2 upon opening of the first air control valve and then the second air control valve is relatively small, it is further difficult to determine the abnormality in the component, for example, the second air control valve based on the pressure difference ΔP2 compared with the abnormality diagnosis with respect to other components.
The secondary air is supplied to the exhaust system in the internal combustion engine through the joint portion and the respective branch portions of the secondary air supply pipe upon activation of the air pump. The discharging performance of the pipe may be deteriorated owing to aging for an extended service period. In case of deterioration in the performance of the pump, the pressure P0 after opening of the auxiliary air control valve by driving the air pump is decreased. Each of the pressure differences ΔP1 and ΔP2 is decreased accordingly. It may further be difficult to determine the abnormality in the component, especially the second air control valve based on the pressure differences ΔP1, ΔP2.